ANODE ACTIVE MATERIAL FOR LITHIUM SECONDARY BATTERY AND LITHIUM SECONDARY BATTERY INCLUDING THE SAME

Abstract

 An anode active material for a secondary battery according to an embodiment of the present invention includes a plurality of a composite particle. The composite particle includes a carbon-based particle containing pores therein, a silicon-containing coating layer formed at an inside of the pores and/or on a surface of the carbon-based particle, and a surface oxide layer formed on the silicon-containing coating layer. The surface oxide layer contains silicon oxide. A silicon oxidation number ratio defined by Equation 1 of the composite particle is 0.6 or less.

Description

BACKGROUND

1. Field

[0001] The present invention relates to an anode active material for a lithium secondary battery, a lithium secondary battery including the same, and a method of preparing an anode active material for a lithium secondary battery.

2. Description of the Related Art

[0002] A secondary battery which can be charged and discharged repeatedly has been widely employed as a power source of a mobile electronic device such as a camcorder, a mobile phone, a laptop computer, etc., according to developments of information and display technologies. Recently, a battery pack including the secondary battery has been developed and applied as a power source for an eco-friendly vehicle such as an electric vehicle.

[0003] The secondary battery includes, e.g., a lithium secondary battery, a nickel-cadmium battery, a nickel-hydrogen battery, etc. The lithium secondary battery is highlighted due to high operational voltage and energy density per unit weight, a high charging rate, a compact dimension, etc.

[0004] For example, the lithium secondary battery may include an electrode assembly including a cathode, an anode and a separation layer (separator), and an electrolyte immersing the electrode assembly. The lithium secondary battery may further include an outer case having, e.g., a pouch shape for accommodating the electrode assembly and the electrolyte.

[0005] Recently, as an application range of the lithium secondary battery is expanded, the lithium secondary battery having higher capacity and power has been researched. For example, a composite of high-capacitance silicon and carbon may be used as an anode active material.

[0006] However, the silicon-carbon composite anode active material has a large volume expansion difference to cause cracks of the anode active material and exposure to an electrolyte solution during repeated charging and discharging.

[0007] Accordingly, an anode active material capable of maintaining capacity properties while suppressing the cracks of the anode active material is required. For example, Korean Registered Patent Publication No. 10-1591698 discloses an anode active material containing silicon oxide, which may not provide sufficient life-span and power properties.

SUMMARY

[0008] According to an aspect of the present invention, there is provided an anode active material for a lithium secondary battery having improved power property and capacity efficiency.

[0009] According to an aspect of the present invention, there is provided a secondary battery including an anode active material with improved power property and capacity efficiency.

[0010] According to exemplary embodiments, an anode active material for a lithium secondary battery comprises a plurality of a composite particle. The composite particle comprises a carbon-based particle containing pores therein, a silicon-containing coating layer formed at an inside of the pores and/or on a surface of the carbon-based particle, and a surface oxide layer formed on the silicon-containing coating layer. The surface oxide layer contains silicon oxide. A silicon oxidation number ratio defined by Equation 1 of the composite particle is 0.6 or less.

[0011] In Equation 1, O_B is an oxidation number of silicon included in the silicon-containing coating layer measured by an X-ray photoelectron spectroscopy (XPS), and O_S is an oxidation number of silicon included in the surface oxide layer measured by the XPS.

[0012] In some embodiments, O_B may be obtained by substituting a value obtained by subtracting 99.6 eV from a binding energy of silicon included in the silicon-containing coating layer measured by the XPS into a silicon oxidation number calibration curve. O_S may be obtained by substituting a value obtained by subtracting 99.6 eV from a binding energy of silicon included in the surface oxide layer measured by the XPS into the silicon oxidation number calibration curve.

[0013] In some embodiments, the silicon oxidation number calibration curve may be obtained by connecting points corresponding to Si⁰, Si¹⁺, Si²⁺, Si³⁺ and Si⁴⁺ with a shortest distance between neighboring points in a graph in which an x-axis represents the oxidation number of silicon and a y-axis represents the value obtained by subtracting 99.6 eV from the binding energy of silicon measured by the XPS.

[0014] In some embodiments, a distance between a surface of the composite particle and the silicon-containing coating layer may be 100 nm to 700 nm, and a distance between the surface of the composite particle and the surface oxide layer may be 10 nm or less.

[0015] In some embodiments, O_B may be in a range from 1.2 to 2.0 and O_S may be in a range from 3.0 to 3.6.

[0016] In some embodiments, an oxygen content ratio defined by Equation 2 is 0.4 or less.

[0017] In Equation 2, C_B is a percentage (at%) of the number of oxygen atoms included in the silicon-containing coating layer relative to a sum of the number of atoms included in the silicon-containing coating layer and the surface oxide layer measured by the XPS. C_S is a percentage (at%) of the number of oxygen atoms included in the surface oxide layer relative to the sum of the number of atoms included in the silicon-containing coating layer and the surface oxide layer measured by the XPS.

[0018] In some embodiments, C_B may be in a range from 8 at% to 15 at%, and C_S may be in a range from 15 at% to 34 at%.

[0019] In some embodiments, the carbon-based particle may include at least one selected from the group consisting of an activated carbon, a carbon nanotube, a carbon nanowire, graphene, a carbon fiber, carbon black, graphite, a porous carbon, pyrolyzed cryogel, pyrolyzed xerogel and pyrolyzed aerogel.

[0020] In some embodiments, the carbon-based particle may have an amorphous structure.

[0021] In some embodiments, silicon included in the silicon-containing coating layer may have an amorphous structure or a crystallite size measured by an X-ray diffraction (XRD) analysis of 7 nm or less.

[0022] In some embodiments, the crystallite size of silicon included in the silicon-containing coating layer may be measured by Equation 3.

[0023] In Equation 3, L represents the crystallite size (nm), λ represents an X-ray wavelength (nm), β represents a full width at half maximum (rad) of a peak corresponding to a (111) plane of silicon contained in the silicon-containing coating layer, and θ represents a diffraction angle (rad).

[0024] In some embodiments, silicon included in the silicon-containing coating layer may have a peak intensity ratio of 1.2 or less in a Raman spectrum defined by Equation 4.

[0025] In Equation 4, I(515) is a peak intensity of silicon included in the silicon-containing coating layer in a region of 515 cm^-1 wavenumber in the Raman spectrum, and I(480) is a peak intensity of silicon included in the silicon-containing coating layer in a region of 480 cm^-1 wavenumber in the Raman spectrum.

[0026] In some embodiments, the composite particle may further comprise a carbon coating layer formed on an outermost portion of the composite particle.

[0027] According to exemplary embodiments, a lithium secondary battery comprises an anode comprising an anode active material layer that comprises the anode active material for a lithium secondary battery according to the above-described embodiments, and a cathode facing the anode.

[0028] According to exemplary embodiments, in a method of preparing an anode active material for a lithium secondary battery, a carbon-based particle including pores is fired together with a silicon source to form a silicon-containing coating layer at an inside of the pores and/or on a surface of the carbon-based particle. The carbon-based particle on which the silicon-containing coating layer is formed is heat-treated while injecting an oxygen gas to form a composite particle including a surface oxide layer formed on the silicon-containing coating layer. The surface oxide layer contains silicon oxide. A silicon oxidation number ratio defined by Equation 1 of the composite particles is 0.6 or less.

[0029] In Equation 1, O_B is an oxidation number of silicon included in the silicon-containing coating layer measured by an X-ray photoelectron spectroscopy (XPS), and O_S is an oxidation number of silicon included in the surface oxide layer measured by the XPS.

[0030] In some embodiments, the heat-treatment may be performed at a temperature from 100 °C to 250 °C to form the surface oxide layer.

[0031] In an aspect of the present invention, an anode cathode active material for a lithium secondary battery prepared by a method that includes firing a carbon-based particle including pores together with a silicon source to form a silicon-containing coating layer at an inside of the pores and/or on a surface of the carbon-based particle; and heat-treating the carbon-based particle on which the silicon-containing coating layer is formed while injecting an oxygen gas to form a composite particle including a surface oxide layer formed on the silicon-containing coating layer is provided.

[0032] In an aspect of the present invention, a lithium secondary battery including the anode active material prepared by the above-described method is provided.

[0033] In one embodiments of the present application, an anode material for a lithium secondary battery includes a plurality of composite particles. At least one of the composite particles includes a carbon-based particle containing pores therein, a silicon-containing coating layer formed inside the pores, and a surface oxide layer formed on the silicon-containing coating layer inside the pores of the carbon-based particle, the surface oxide layer containing silicon oxide. A distance between a surface of the composite particle and the silicon-containing coating layer is 100 nm to 700 nm, and a distance between the surface of the composite particle and the surface oxide layer formed on the silicon-containing coating layer is 10 nm or less.

[0034] In one embodiments of the present application, an anode material for a lithium secondary battery includes a plurality of composite particles. At least one of the composite particles includes a carbon-based particle containing pores therein, a silicon-containing coating layer formed inside the pores or on a surface of the carbon-based particle, and a surface oxide layer formed on the silicon-containing coating layer, the surface oxide layer containing silicon oxide. silicon included in the silicon-containing coating layer has an amorphous silicon structure or has a crystallite size measured by an X-ray diffraction (XRD) analysis of 7 nm or less, and silicon in the silicon dioxide has an oxidation state ranging from 2.9 to 3.6.

[0035] In one embodiments of the present application, an anode material for a lithium secondary battery includes a plurality of composite particles. At least one of the composite particles includes an amorphous carbon-based particle containing pores therein having a pore size of 20 nm or less, a silicon-containing coating layer formed inside the pores of the amorphous carbon-based particle, and a surface oxide layer formed on the silicon-containing coating layer, the surface oxide layer containing silicon oxide. The pore size of 20 nm or less restricts a quantity of silicon contained in the amorphous carbon-based particle to thereby reduce cracking in the anode material during repeated charging and discharging cycles.

[0036] According to the present invention, carbon-based particles include pores. For example, the carbon-based particle may be a porous particle including a plurality of pores. A silicon-containing coating layer is formed on at least one of an inside and a surface of the pores. Accordingly, cracks due to a difference in volume expansion ratio between carbon and silicon during charging and discharging of the secondary battery may be prevented.

[0037] A surface oxide layer containing silicon oxide is formed on the silicon-containing coating layer. An oxidation number of silicon included in the surface oxide layer may be greater than an oxidation number of silicon included in the silicon-containing coating layer. Thus, silicon element included in the silicon-containing coating layer may be prevented from reacting with moisture in an air or a solvent (e.g., water) of a slurry to generate gas. Additionally, transformation of the silicon element of the silicon-containing coating layer into silicon oxide may be suppressed, thereby preventing deterioration of capacity properties of an anode active material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] 

FIG. 1 is a schematic graph showing a silicon oxidation number calibration curve.

FIGS. 2 and 3 are a schematic plan view and a schematic cross-sectional view illustrating a secondary battery in accordance with exemplary embodiments.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0039] According to embodiments of the present invention, an anode active material for a secondary battery including a carbon-based particle and a silicon-containing coating layer is provided. According to embodiments of the present invention, a lithium secondary battery including the anode active material is also provided. Further, according to embodiments of the present invention a method of preparing an anode active material for a lithium secondary battery is provided.

[0040] Hereinafter, detailed descriptions of the present invention will be described in detail with reference to exemplary embodiments. However, those skilled in the art will appreciate that such embodiments are provided to further understand the present invention and do not limit subject matters to be protected as disclosed in the detailed description and defined by the appended claims.

[0041] For example, the anode active material may be formed to include both silicon and a carbon-based particle. The anode active material may include a plurality of the carbon-based particle. In this case, carbon may partially mitigate a volume expansion of silicon. However, during charging and discharging of a secondary battery, a difference between a volume expansion ratio of silicon (e.g., about 400 % or more) and a volume expansion ratio of carbon (e.g., about 150 % or less) increases, resulting in cracks in the anode active material. Accordingly, when charging and discharging are repeated, the anode active material may be exposed to an electrolyte thereby causing side reactions such as a gas generation and degrading life-span properties of the secondary battery.

[0042] The carbon-based particle includes pores. For example, the carbon-based particle may be a porous particle including a plurality of pores therein.

[0043] A silicon-containing coating layer is formed on at least one of an inside of the pores and a surface of the carbon-based particle. Accordingly, cracks due to the difference of the volume expansion ratios between carbon and silicon may be prevented during charging and discharging.

[0044] In exemplary embodiments, the carbon-based particle may have a pore size of 20 nm or less, and preferably, less than 10 nm. Within this range, excessive deposition of silicon in the pores may be prevented. Accordingly, defects caused by the difference of the volume expansion ratio between carbon and silicon during charging and discharging of the secondary battery may be further suppressed.

[0045] In some embodiments, the pore size of the carbon-based particle may be in a range from 0.1 nm to 20 nm, or from 0.1 nm to 10 nm.

[0046] For example, the above-mentioned carbon-based particle may include an activated carbon, a carbon nanotube, a carbon nanowire, graphene, a carbon fiber, carbon black, graphite, a porous carbon, pyrolyzed cryogel, pyrolyzed xerogel, pyrolyzed aerogel, etc. These may be used alone or in a combination thereof.

[0047] In some embodiments, the carbon-based particle may have an amorphous structure or a crystalline structure. Preferably, the carbon-based particles may include an amorphous structure. Accordingly, durability of the anode active material may be increased, and generation of cracks by the charge/discharge or the external impact may be suppressed. Thus, the life-span properties of the secondary battery may be improved.

[0048] The anode active material includes a silicon-containing coating layer formed at an inside of the pores of the carbon-based particle and/or on a surface of the carbon-based particle. The difference of the volume expansion ratio between carbon and silicon may be alleviated while employing high capacitance properties of silicon included in the silicon-containing coating layer. Thus, micro-cracks and exposure to an electrolyte caused by repeated charging and discharging of the secondary battery may be reduced, and the life-span properties of the secondary battery may be improved while maintaining power properties.

[0049] For example, the silicon-containing coating layer may refer to a layer in which silicon particles are formed on at least a portion of the pores and/or the surface of carbon-based particle.

[0050] A surface oxide layer containing silicon oxide is formed on the silicon-containing coating layer. In an embodiment, the silicon particles located on the surface of the silicon-containing coating layer may be oxidized to form the surface oxide layer.

[0051] For example, silicon oxide may be SiOx (0<x≤2).

[0052] An anode active material for a lithium secondary battery according to embodiments of the present invention comprises a composite particle comprising the above-described carbon-based particle, the silicon-containing coating layer and the surface oxide layer. For example, the silicon-containing coating layer may be formed directly at the inside of the pores and/or on the surface of the pores. For example, the silicon-containing coating layer may be disposed under the surface oxide layer.

[0053] In an embodiment, the silicon-containing coating layer may include an inner portion of the composite particle including a distance of 100 nm to 700 nm from a surface of the composite particle, but a position of the silicon-containing coating layer is not limited to the above range.

[0054] For example, the surface oxide layer may refer to an outermost portion having a distance of 10 nm or less from the surface of the composite particle.

[0055] For example, a distance between a surface of the composite particle and the silicon-containing coating layer may be 100 nm to 700 nm, and a distance between the surface of the composite particle and the surface oxide layer may be 10 nm or less.

[0056] The term "silicon-containing coating layer" as used herein may represent a region in which a distance from an outer surface of the composite particle is 100 nm to 700 nm.

[0057] The term "surface oxide layer" as used herein may represent a region in which a distance from the outer surface of the composite particle is 0 nm to 10 nm.

[0058] In various embodiments, the composite particle may further include an intermediate region disposed in a region between the silicon-containing coating layer and the surface oxide layer.

[0059] The term "intermediate region" as used herein may represent a region in which a distance from the outer surface of the composite particle is greater than 10 nm and less than 100 nm.

[0060] The intermediate region may include silicon, silicon oxide, or both silicon and silicon oxide.

[0061] For example, an intermediated layer may be disposed between the silicon-containing coating layer and the surface oxide layer. The intermediate layer may include both silicon and silicon oxide together.

[0062] For example, a side reaction between a silicon element and water may be suppressed by the surface oxide layer, and the life-span properties of the secondary battery may be improved. However, if an excessive amount of silicon is transformed into silicon oxide, the capacity properties of the anode active material may be degraded.

[0063] In exemplary embodiments, an oxidation number of silicon included in the surface oxide layer may be greater than an oxidation number of silicon included in the silicon-containing coating layer. For example, the surface oxide layer may serve as a protective layer for the silicon-containing coating layer. Accordingly, the silicon element included in the silicon-containing coating layer may be prevented from reacting with moisture in an air or a solvent (e.g., water) of the slurry to generate gas. Additionally, transformation of the silicon element of the silicon-containing coating layer into silicon oxide may be suppressed, thereby preventing an excessive reduction of the capacity properties of an anode active material.

[0064] The term "oxidation number" as used herein indicate the number of charge of a specific atom constituting a material assuming that an exchange of electrons has completely occurs in the material (a molecule, an ionic compound, a simple substance, etc.). For example, the oxidation number may indicate an oxidation state.

[0065] For example, the oxidation number of Si⁰ is 0, the oxidation number of Si¹⁺ is +1, the oxidation number of Si²⁺ is +2, the oxidation number of Si³⁺ is +3, and the oxidation number of Si⁴⁺ is +4. The "+" notation in front of the oxidation number can be omitted. For example, the oxidation number of Si¹⁺ can be expressed as 1.

[0066] A silicon oxidation number ratio defined by Equation 1 below of the composite particle is 0.6 or less. Preferably, the silicon oxide number ratio of the composite particles may be in a range from 0.01 to 0.5.

[0067] In Equation 1, O_B is an oxidation number of silicon included in the silicon-containing coating layer obtained through an X-ray photoelectron spectroscopy (XPS), and O_S is an oxidation number of silicon included in the surface oxide layer obtained through the XPS.

[0068] FIG. 1 is a schematic graph showing a silicon oxidation number calibration curve.

[0069] Referring to FIG. 1, the silicon oxidation number calibration curve may be obtained as follows.

[0070] A graph with the oxidation number of silicon as an x-axis and a value obtained by subtracting 99.6 eV from a binding energy of silicon measured through the XPS (e.g., Δ (binding energy)) as the y-axis may be set.

[0071] In the graph above, points corresponding to Si⁰, Si¹⁺, Si²⁺, Si³⁺ and Si⁴⁺ may be denoted on the graph, and then the silicon oxide number calibration curve may be obtained by connecting adjacent points with the shortest distance.

[0072] For example, the binding energies of Si⁰, Si¹⁺, Si²⁺, Si³⁺ and Si⁴⁺ measured through the XPS may be 99.6 eV, 100.6 eV, 101.4 eV, 102.2 eV and 103.7 eV, respectively. Thus, the points corresponding to Si⁰, Si¹⁺, Si²⁺, Si³⁺ and Si⁴⁺ has (x, y) coordinates of (0,0), (1,1), (2,1.8), (3,2.6) and (4,4.1), respectively.

[0073] In Equation 1, O_B may be obtained by substituting a value obtained by subtracting 99.6 eV, which is a binding energy of Si⁰, from a binding energy of silicon included in the silicon-containing coating layer measured by the XPS into the silicon oxidation number calibration curve.

[0074] In Equation 1, O_S may be obtained by substituting a value obtained by subtracting 99.6 eV from the binding energy of silicon included in the surface oxide layer measured by the XPS into the silicon oxidation number calibration curve.

[0075] For example, each binding energy of silicon included in the silicon-containing coating layer and silicon included in the surface oxide layer may be measured through the XPS. Y values of the silicon oxidation number calibration curve described above may be obtained by subtracting 99.6 eV from each of the measured binding energies. As indicated by arrows in FIG. 1, oxidation numbers of silicon (e.g., O_B and O_S) may be obtained from x values corresponding to the y values.

[0076] In an embodiment, the binding energy of the silicon included in the silicon-containing coating layer may be obtained by etching to a depth of 100 nm or more from the surface of the composite particle using an argon (Ar) ion gun (an Ar monatomic gun).

[0077] The indicator line (the arrow) in FIG. 1 is provided to explain the process of obtaining the oxidation number of silicon as an example.

[0078] Within the range of the silicon oxide number ratio, excessive transformation of silicon in the silicon-containing coating layer into silicon oxide may be prevented while sufficiently including silicon oxide included in the surface oxide layer. Thus, the side reactions between silicon and water may be suppressed to maintain the capacity properties and improve the life-span properties.

[0079] In some embodiments, O_B may be in a range from 1.2 to 2.0, and O_S may be in a range from 3.0 to 3.6. Preferably, O_B may be in a range from 1.3 to 1.8 and O_S may be in a range from 3.1 to 3.5. Within the above range, side reactions occurring in the surface oxide layer may be suppressed while suppressing the excessive oxidation of silicon included in the silicon-containing coating layer.

[0080] In exemplary embodiments, an oxygen content of the surface oxide layer measured through the XPS may be greater than an oxygen content of the silicon-containing coating layer measured through the XPS. Accordingly, gas generation due to a contact between silicon and water as a slurry solvent or moisture in an air may be suppressed by the surface oxide layer.

[0081] In exemplary embodiments, an oxygen content ratio defined by Equation 2 below may be 0.4 or less, preferably in a range from 0.01 to 0.35.

[0082] In Equation 2, C_B is a percentage (at%) of the number of oxygen atoms included in the silicon-containing coating layer relative to a sum of the number of atoms included in the silicon-containing coating layer and the number of atoms included in the surface oxide layer measured by the XPS. C_S is a percentage (at%) of the number of oxygen atoms included in the surface oxide layer relative to the sum of the number of atoms included in the silicon-containing coating layer and the number of atoms included in the surface oxide layer measured by the XPS.

[0083] For example, the number of atoms included in the silicon-containing coating layer and the number of atoms included in the surface oxide layer may refer to the number of all atoms included in the silicon-containing coating layer and the number of all atoms included in the surface oxide layer, respectively.

[0084] Within the above oxygen content ratio range, the oxidation of silicon included in the silicon-containing coating layer may be suppressed while sufficiently suppressing the above-described side reaction of silicon included in the composite particle. Accordingly, the side reactions between silicon and water may be suppressed to maintain the capacity properties and improve the life-span properties.

[0085] In some embodiments, C_B may be in a range from 8 at% to 15 at%, and C_S may be in a range from 15 at% to 34 at%. Preferably, C_B may be in a range from 9 at% to 12 at%, and C_S may be in a range from 30 at% to 33 at%. Within this range, the surface oxide layer may be sufficiently formed while suppressing a formation of an oxide layer on the silicon-containing coating layer.

[0086] In exemplary embodiments, the above-described silicon-containing coating layer may at least partially have an amorphous structure or may contain silicon having a crystallite size of 7 nm or less as measured by an X-ray diffraction (XRD) analysis. In a preferable embodiment, the crystallite size may be 4 nm or less. Within the above range, mechanical stability of the anode active material may be improved during the press process for manufacturing the lithium secondary battery or during the repeated charging and discharging. Accordingly, a capacity retention may be increased to improve the life-span properties the lithium secondary battery may be improved.

[0087] The term "amorphous structure" as used herein refers to a case that a shape of a silicon particle included in the silicon-containing coating layer is amorphous or a case that a crystallite size is excessively small and may not be measured through a Scherrer equation represented by Equation 3 using the X-ray diffraction (XRD) analysis.

[0088] In Equation 3 above, L represents the crystallite size (nm), λ represents an X-ray wavelength (nm), β represents a full width at half maximum (FWHM) of a peak, and θ represents a diffraction angle (rad). In exemplary embodiments, the FWHM in the XRD analysis for measuring the crystallite size may be measured from a peak of (111) plane of silicon contained in the silicon-containing coating layer.

[0089] In some embodiments, in Equation 3 above, β may represent a FWHM obtained by correcting a value derived from an equipment. In an embodiment, Si may be used as a standard material for reflecting the equipment-derived value. In this case, the device-derived FWHM may be expressed as a function of 2θ by fitting a FWHM profile in an entire 2θ range of Si. Thereafter, a value obtained by subtracting and correcting the equipment-derived FWHM value at the corresponding 2θ from the function may be used as β.

[0090] In some embodiments, the silicon-containing coating layer may further contain at least one of SiOx (0<x<2) and silicon carbide (SiC).

[0091] In some embodiments, silicon carbide may not be formed at the inside of the pores or on the surface of the carbon-based particle. For example, the silicon-containing coating layer may not include silicon carbide. For example, the silicon-containing coating layer may contain only silicon and/or silicon oxide. Accordingly, the capacity properties of the secondary battery may be improved.

[0092] For example, the formation of silicon carbide may be suppressed by adjusting a temperature and a time of a silicon deposition.

[0093] In some embodiments, the above-described silicon may include an amorphous structure. In this case, the crystallite size of silicon and a peak intensity ratio of a Raman spectrum as described later may be maintained within an appropriate range. Accordingly, improved life-span properties may be achieved while maintaining the capacity properties.

[0094] In exemplary embodiments, a peak intensity ratio from a Raman spectrum of silicon included in the silicon-containing coating layer defined by Equation 4 below may be 1.2 or less, preferably 1.0 or less.

[0095] In Equation 4, I(515) is a peak intensity of silicon included in the silicon-containing coating layer in a region corresponding to a wave number of 515 cm^-1 in the Raman spectrum, and I(480) is a peak intensity of silicon included in the silicon-containing coating layer in a region corresponding to a wave number of 480 cm^-1 in the Raman spectrum.

[0096] For example, I(515) in Equation 4 may represent a portion of silicon having a crystalline structure included in the silicon-containing coating layer, and I(480) in Equation 4 may represent a portion of silicon having an amorphous structure included in the silicon-containing coating layer.

[0097] In the peak intensity ratio range, the amorphous structure ratio of silicon included in the silicon-containing coating layer may be increased, so that structural stability of the anode active material may be improved. Accordingly, the life-span properties of the secondary battery may be improved.

[0098] In some embodiments, the above-described crystallite size range and the peak intensity ratio range from the Raman spectrum of silicon included in the silicon-containing coating layer may be both satisfied. Accordingly, the amorphous properties of the silicon-containing coating layer may be further improved, and stability of the anode active material may also be improved. Thus, the life-span properties of the anode active material may be further improved.

[0099] In some embodiments, a carbon coating layer may be further formed on an outermost portion of the composite particle. Accordingly, a contact between silicon of the anode active material and moisture in the air or a contact between silicon and water in the anode slurry may be prevented. Thus, a reduction of a discharge capacity and capacity efficiency of the secondary battery may be suppressed during a period from a preparation of the anode active material to a formation of the anode.

[0100] For example, the carbon coating layer may refer to a layer in which carbon particles are formed on at least a portion of the silicon-containing coating layer and/or the surface oxide layer.

[0101] In some embodiments, the carbon coating layer may include at least one of carbon and a conductive polymer. Accordingly, the above-described effect of preventing the contact between water and silicon may be implemented while facilitating the formation of the carbon coating layer. Thus, the reduction of the discharge capacity and capacity efficiency of the secondary battery may be suppressed.

[0102] For example, the conductive polymer may include polyacetylene, polyaniline, polypyrrole and/or polythiophene.

[0103] In some embodiments, the carbon coating layer may also be formed on a portion of the inside and the surface of the pores of the carbon-based particle on which the silicon-containing coating layer or the surface oxide layer is not formed. For example, the carbon coating layer may entirely cover silicon, silicon oxide and the carbon-based particle on the composite particle including the silicon-containing coating layer and the surface oxide layer formed thereon. Thus, mechanical and chemical stability of the anode active material may be improved while preventing the contact between silicon and water.

[0104] Hereinafter, a method of preparing the anode active material according to exemplary embodiments, preferably the anode active material for a lithium secondary battery according to the above-described embodiments, is described in more detail.

[0105] A carbon-based particle including pores may be prepared.

[0106] In some embodiments, a resol oligomer may be prepared by mixing an aromatic compound containing a hydroxyl group with an aldehyde-based compound. For example, the aromatic compound including the hydroxyl group may include phenol, and the aldehyde-based compound may include formaldehyde. The resol oligomer may be cured by adding a curing agent, and then the carbon-based particles including pores may be obtained by classification, washing with water and firing.

[0107] In some embodiments, an aromatic compound and a vinyl-based compound may be mixed and polymerized. Thereafter, the carbon-based particles including pores may be obtained by washing with water and firing. For example, the aromatic compound may include polystyrene, and the vinyl-based compound may include divinylbenzene.

[0108] In some embodiments, an activation process may be performed. In this case, an activity of a pore structure in the carbon-based particles may be easily controlled.

[0109] In an embodiment, the activation process may include a physical activation method. For example, a gas having a reactivity with carbon (a steam, a carbon dioxide gas, or a mixed gas of the steam, the carbon dioxide gas and an inert gas) may be introduced, and a heat treatment may be performed at a temperature from 700° C to 1000 °C.

[0110] In an embodiment, the activation process may include a chemical activation method. For example, an acidic or basic chemical such as KOH, Na₂CO₃, NaOH and H₃PO₄ may be used as an activator. The chemical activation method may be performed at a lower temperature than that of the physical activation method.

[0111] Size of pores included in the carbon-based particle obtained by the above method may be 20 nm or less.

[0112] In exemplary embodiments, a silicon source may be injected into a reactor where the carbon-based particles are loaded, and then a firing may be performed to form the silicon-containing coating layer at the inside of the pores and/or the surface of the carbon-based particle.

[0113] For example, the silicon source may include a silicon-containing gas. In an embodiment, the silicon-containing gas may be a silane gas.

[0114] In some embodiments, the firing may be performed at a temperature less than 600 °C, preferably from 100 °C to 300 °C. Within the above temperature range, silicon having the amorphous structure may be sufficiently included in the silicon-containing coating layer. Accordingly, mechanical stability of the anode active material may be improved during a press process or repeated charging and discharging of the secondary battery.

[0115] In exemplary embodiments, the surface oxide layer may be formed by performing a heat treatment while injecting an oxygen gas into the carbon-based particle on which the silicon-containing coating layer is formed. For example, the surface oxide layer containing silicon oxide may be formed by oxidizing silicon particles included in a surface portion of the silicon-containing coating layer.

[0116] The silicon-containing coating layer and the surface oxide layer may have a silicon oxide number ratio defined by Equation 1 of 0.6 or less, preferably from 0.01 to 0.5.

[0117] In an embodiment, the surface oxide layer may be formed by depositing silicon in the pores of the carbon-based particles and/or on the surface of the carbon-based particles, and then oxidizing silicon by an oxygen gas injection and a heat treatment. Accordingly, the surface portion of the composite particle may be sufficiently protected by silicon oxide, and the silicon-containing coating layer may maintain high-capacity properties of silicon.

[0118] According to some embodiments, the heat treatment for the formation of the surface oxide layer may be performed at a temperature from 100 °C to 250 °C. Within the above range, the side reactions of silicon may be suppressed while preventing the excessive oxidation of silicon.

[0119] In some embodiments, the carbon coating layer may be formed on the surface oxide layer by introducing a carbon source into the reactor. For example, the composite particle may include the carbon coating layer formed on an outermost portion.

[0120] For example, the carbon source may include pitch, glucose, sucrose, a phenolic hydrocarbon and/or a resorcinol-based hydrocarbons. In this case, firing may be performed after the introduction of the carbon source.

[0121] In some embodiments, the carbon source may include a methane gas, an ethylene gas, a propylene gas, an acetylene gas, etc. These may be used alone or in a combination thereof. In this case, the carbon coating layer may be formed by a chemical vapor deposition (CVD). For example, the chemical vapor deposition may be performed by firing the carbon source.

[0122] In some embodiments, the carbon source may be a conductive polymer including at least one of polyacetylene, polyaniline, polypyrrole and polythiophene. In this case, the carbon coating layer may be formed by a chemical vapor deposition, an electro-polymerization or a solution process.

[0123] In an embodiment, the conductive polymer may be modified into carbon by a firing after coating the conductive polymer.

[0124] In some embodiments, the firing for forming the carbon coating layer may be performed at a temperature less than 600 °C. In the above temperature range, a ratio of carbon included in the carbon coating layer and silicon included in the silicon-containing coating layer having the amorphous structure may be sufficiently increased. Accordingly, mechanical stability of the anode active material may be improved during the press process or repeated charging and discharging of the secondary battery.

[0125] FIGS. 2 and 3 are a schematic plan view and a schematic cross-sectional view, respectively, illustrating a secondary battery according to exemplary embodiments. For example, FIG. 3 is a cross-sectional view taken along a line I-I' in FIG. 2 in a thickness direction of the lithium secondary battery.

[0126] Referring to FIGS. 2 and 3, a lithium secondary battery may include an electrode assembly including an anode 130, a cathode 100 and a separation layer 140 interposed between the cathode and the anode. The electrode assembly may be accommodated and impregnated with an electrolyte in a case 160.

[0127] The cathode 100 may include a cathode active material layer 110 formed by coating a mixture containing a cathode active material on a cathode current collector 105.

[0128] The cathode current collector 105 may include aluminum, stainless steel, nickel, titanium, or an alloy thereof, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc.

[0129] The cathode active material may include a compound capable of reversibly intercalating and de-intercalating lithium ions.

[0130] In exemplary embodiments, the cathode active material may include a lithium-transition metal oxide. For example, the lithium-transition metal oxide includes nickel (Ni) and may further include at least one of cobalt (Co) and manganese (Mn).

[0131] For example, the lithium-transition metal oxide may be represented by Chemical Formula 1 below.

         [Chemical Formula 1]     Li_xNi_1-yM_yO_2+z

[0132] In Chemical Formula 1, 0.9≤x≤1.2, 0≤y≤0.7, and -0.1≤z≤0.1. M may include at least one element selected from Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Co, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn and Zr.

[0133] In some embodiments, a molar ratio or a concentration (1-y) of Ni in Chemical Formula 1 may be greater than or equal to 0.8, and may exceed 0.8 in preferable embodiment.

[0134] The mixture may be prepared by mixing and stirring the cathode active material in a solvent with a binder, a conductive material and/or a dispersive agent. The mixture may be coated on the cathode current collector 105, and then dried and pressed to form the cathode 100.

[0135] The solvent may include a non-aqueous solvent. For example, N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N,N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, etc., may be used.

[0136] For example, the binder may include an organic based binder such as a polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidenefluoride (PVDF), polyacrylonitrile, polymethylmethacrylate, etc., or an aqueous based binder such as styrene-butadiene rubber (SBR) that may be used with a thickener such as carboxymethyl cellulose (CMC).

[0137] For example, a PVDF-based binder may be used as a cathode binder. In this case, an amount of the binder for forming the cathode active material layer may be reduced, and an amount of the cathode active material may be relatively increased. Thus, capacity and power of the lithium secondary battery may be further improved.

[0138] The conductive material may be included to promote an electron movement between active material particles. For example, the conductive material may include a carbon-based conductive material such as graphite, carbon black, graphene, carbon nanotube, etc., and/or a metal-based conductive material such as tin, tin oxide, titanium oxide, a perovskite material such as LaSrCoO₃, LaSr4MnO₃, etc.

[0139] In exemplary embodiments, an anode slurry may be prepared from the above-described anode active material including the composite particle. For example, the anode slurry may be prepared by mixing and stirring the anode active material with an anode binder, a conductive material and a thickener in a solvent.

[0140] For example, the anode active material may include a plurality of the composite particles.

[0141] For example, the anode active material may include a plurality of the composite particles and a graphite-based active material. For example, the graphite-based active material may include artificial graphite and/or natural graphite.

[0142] An amount of the composite particles based on a total weight of the anode active material (e.g., the total weight of the composite particles and the graphite-based active material) may be 3 wt% or more, 5 wt% or more, 10 wt% or more, 15 wt% or more, 20 wt% or more, 25 wt% or more, 30 wt% or more, 35 wt% or more, 40 wt% or more, or 45 wt% or more.

[0143] The amount of the composite particles based on the total weight of the anode active material may be 90 wt% or less, 85 wt% or less, 80 wt% or less, 75 wt% or less, 70 wt% or less, 65 wt% or less, 60 wt% or less, 55 wt% or less, or 50 wt% or less.

[0144] In an embodiment, the anode active material may substantially consist of the composite particles and the graphite-based active material.

[0145] For example, the solvent included in the anode slurry may be an aqueous solvent such as water, an aqueous hydrochloric acid solution, or an aqueous sodium hydroxide solution, etc.

[0146] For example, the anode binder may include a polymer material such as styrene-butadiene rubber (SBR). Examples of the thickener include carboxymethyl cellulose (CMC).

[0147] For example, the conductive material may include a material of the same type as that of the above-described conductive material included for forming the cathode active material layer.

[0148] In some embodiments, the anode 130 may include an anode active material layer 120 formed by applying (coating) the above-described anode slurry on at least one surface of an anode current collector 125 and then drying and pressing the anode slurry.

[0149] For example, the anode current collector 125 may include a metal that has high conductivity, and may be easily adhered to the anode slurry and non-reactive within a voltage range of the battery. For example, stainless steel, nickel, copper, titanium, an alloy thereof, or copper or stainless steel surface-treated with carbon, nickel, titanium or silver may be used.

[0150] The separation layer 140 may be interposed between the cathode 100 and the anode 130. The separation layer 140 may include a porous polymer film prepared from, e.g., a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, an ethylene/methacrylate copolymer, or the like. The separation layer 140 may be also formed from a non-woven fabric including a glass fiber with a high melting point, a polyethylene terephthalate fiber, or the like.

[0151] In some embodiments, an area and/or a volume of the anode 130 (e.g., a contact area with the separation layer 140) may be greater than that of the cathode 100. Thus, lithium ions generated from the cathode 100 may be easily transferred to the anode 130 without loss by, e.g., precipitation or sedimentation. Thus, capacity and power of the lithium secondary battery may be improved.

[0152] In exemplary embodiments, an electrode cell may be defined by the cathode 100, the anode 130 and the separation layer 140, and a plurality of the electrode cells may be stacked to form the electrode assembly 150 having, e.g., a jelly roll shape. For example, the electrode assembly 150 may be formed by winding, laminating or folding of the separation layer 140.

[0153] The electrode assembly 150 may be accommodated together with an electrolyte in the case 160 to define the lithium secondary battery. In exemplary embodiments, a non-aqueous electrolyte may be used as the electrolyte.

[0154] For example, the non-aqueous electrolyte may include a lithium salt and an organic solvent. The lithium salt and may be represented by Li⁺X^-. An anion of the lithium salt X' may include, e.g., F^-, Cl^-, Br^-, I^-, NO₃^-, N(CN)₂^-, BF₄^-, ClO₄^-, PF₆^-, (CF₃)₂PF₄^-, (CF₃)₃PF₃^-, (CF₃)₄PF₂^-, (CF₃)₅PF^-, (CF₃)₆P^-, CF₃SO₃^-, CF₃CF₂SO₃^-, (CF₃SO₂)₂N^-, (FSO₂)₂N^-, CF₃CF₂(CF₃)₂CO^-, (CF₃SO₂)₂CH^-, (SF₅)₃C^-, (CF₃SO₂)₃C^-, CF₃(CF₂)₇SO₃^-, CF₃CO₂^-, CH₃CO₂^-, SCN^-, (CF₃CF₂SO₂)₂N^-, etc.

[0155] The organic solvent may include, e.g., propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxy ethane, diethoxy ethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite, tetrahydrofuran, etc. These may be used alone or in a combination thereof.

[0156] As illustrated in FIG. 2, electrode tabs (a cathode tab and an anode tab) may protrude from the cathode current collector 105 and the anode electrode current collector 125 included in each electrode cell to one side of the case 160. The electrode tabs may be welded together with the one side of the case 160 to form an electrode lead (a cathode lead 107 and an anode lead 127) extending or exposed to an outside of the case 160.

[0157] The lithium secondary battery may be manufactured in, e.g., a cylindrical shape using a can, a square shape, a pouch shape or a coin shape.

[0158] Hereinafter, preferred embodiments are proposed to more concretely describe the present invention. However, the following examples are only given for illustrating the present invention and those skilled in the related art will obviously understand that various alterations and modifications are possible within the scope of the present invention. Such alterations and modifications are duly included in the appended claims.

Example 1

[0159] 

i) Synthesis of resol oligomer: Phenol and formaldehyde were mixed in a molar ratio of 1:2, and 1.5 wt% of triethylamine was added thereto, followed by a reaction under conditions of 85 °C, 4 hours and 160 rpm (stirring).

ii) Suspension stabilization of resol oligomer: 1 g of poly(vinyl alcohol) (PVA) was dispersed in a water-dispersible medium, and then added to the resol oligomer.

iii) Curing of resol oligomer: 3 g of HMTA (hexamethylene tetramine) as a curing agent was added to the resol oligomer, and reacted under conditions of 98 °C, 12 hours and 400 rpm (with stirring).

iv) Obtaining carbon material: The cured resol oligomer was classified using a sieve, and then washed with H₂O.

v) Unreacted monomers and oligomers were removed from the washed resol oligomer using ethanol, and dried.

vi) Carbonization and activation: The dried resol oligomer was calcined at 900 °C for 1 hour under a nitrogen atmosphere. During the firing, CO₂ gas was introduced at 1 L/min and carbonized at 900 °C.

Formation of silicon-containing coating layer

[0160] A silane gas was injected into a CVD coater at a flow rate in a range from 50 mL/min to 100 mL/min, and a temperature was raised at a heating rate of 5 °C/min to 20 °C/min and maintained at 200 °C for about 2 hours to deposit silicon, thereby forming a silicon-containing coating layer.

Formation of surface oxide layer

[0161] A high-concentration oxygen gas was injected into the CVD coater at a flow rate of 50 mL/min to 100 mL/min, and a temperature was raised at a heating rate of 5 °C/min to 20 °C/min and maintained at 100 °C for about 1 hour to oxidize the deposited silicon surface, thereby forming a surface oxide layer.

[0162] Accordingly, an anode active material including composite particles including carbon-based particles, a silicon-containing coating layer and a surface oxide layer was prepared.

Fabrication of anode

[0163] The prepared anode active material was left for one day. 95.5 wt% of a mixture of 15 wt% of the prepared anode active material and 80.5 wt% of artificial graphite, 1 wt% of CNT as a conductive material, 2 wt% of styrene-butadiene rubber (SBR), and 1.5 wt% of carboxymethyl cellulose (CMC) as a thickener were mixed to form an anode slurry.

[0164] The anode slurry was coated on a copper substrate, and dried and pressed to obtain an anode.

Fabrication of Li-half cell

[0165] A lithium secondary battery was manufactured using the anode manufactured as described above and a lithium metal as a counter electrode (cathode).

[0166] Specifically, a lithium coin half-cell was constructed by interposing a separator (polyethylene, thickness of 20 µm) between the prepared anode and the lithium metal (thickness of 1 mm).

[0167] The assembly of lithium metal/separator/cathode was placed in a coin cell plate, an electrolyte was injected, a cap was covered, and then clamped. The electrolyte was prepared by preparing a 1M LiPF₆ solution using a mixed solvent of EC/EMC (3:7; volume ratio), and then adding 2.0 vol% of FEC based on a total volume of the electrolyte. An impregnation for 3 to 24 hours after clamping was performed, and then 3 cycles of charging and discharging at 0.1C were performed (charging condition CC-CV 0.1C 0.01V 0.01C CUT-OFF, discharging condition CC 0.1C 1.5V CUT-OFF).

Examples 2 to 5

[0168] An anode and a lithium half-cell were manufactured by the same method as that in Example 1, except that an oxidation treatment was performed at a temperature shown in Table 2 when forming the surface oxide layer.

Examples 6 and 7

[0169] An anode and a lithium half-cell were manufactured by the same method as that in Example 1, except that the silane gas was injected into the CVD coater at a flow rate of 100 mL/min to 500 mL/min, and a temperature was maintained at 600 °C or higher for about 30 to 120 minutes at a heating rate of 5 °C/min to 20 °C/min to deposit silicon.

[0170] The silicon-containing coating layer included in the anode active material according to Examples 6 and 7 was formed by changing the silane gas flow rate, the heating rate, the temperature and time.

Comparative Example 1

[0171] An anode and a lithium half-cell were manufactured by the same method as that in Example 1, except that the surface oxide layer was not formed.

Comparative Example 2

[0172] An anode and a lithium half-cell were manufactured by the same method as that in Example 1, except that the oxidation treatment was performed at a temperature shown in Table 2 when forming the surface oxide layer.

Experimental Example

(1) Measurement of silicon oxidation number ratio

1) Measurement of binding energy of silicon through XPS

[0173] Each anode prepared according to Examples and Comparative Examples was sampled by being attached to a carbon tape, and an XPS analysis was performed under the following conditions to measure a binding energy of silicon.

[XPS analysis conditions]

[0174] 

i) X-ray type: Al k alpha, 1486.68 eV, 900 µm Beam size

ii) Analyzer: CAE (constant analyzer energy) Mode

iii) Number of scans: 50

iv) Pass energy: 20 eV

v) Dwell Time: 100 ms

vi) Ion gun: Ar ion

vii) Ion energy: 4000 eV

viii) Etch Cycle: 300s

ix) Total Levels: 20

[0175] Specifically, the binding energy of silicon included in the surface oxide layer of the anode active material prepared according to Examples and Comparative Examples was measured by using XPS on the surface oxide layer.

[0176] Further, a binding energy of silicon contained in the silicon-containing coating layer was measured by measuring the binding energy of silicon in a section where the silicon content was not changed according to a depth (e.g., a distance from the particle surface of 100 nm or more) using an Ar monatomic ion gun.

2) Measurement of silicon oxidation number ratio

[0177] A value obtained by subtracting 99.6 eV from the measured binding energies of silicon included in the surface oxide layer and the silicon-containing coating layer, respectively, was substituted as a y value of a silicon oxide number calibration curve to obtain oxidation numbers of silicon (x value of the oxidation number calibration curve) included in the surface oxide layer and the silicon-containing coating layer.

[0178] An oxidation number ratio of silicon was calculated by substituting the obtained oxidation numbers of silicon into Equation 1.

(2) Measurement of oxygen content ratio

[0179] An XPS analysis was performed on the anode active materials prepared according to Examples and Comparative Examples under the same conditions as those described in Experimental Example (1) 1). From the XPS analysis, a percentage (at%) of the number of oxygen atoms included in the surface oxide layer relative to the total number of atoms included in the silicon-containing coating layer and the surface oxide layer, and a percentage (at%) of the number of oxygen atoms included in the silicon-containing coating layer relative to the total number of atoms included in the silicon-containing coating layer and the surface oxide layer were measured.

(3) Raman spectroscopy spectrum analysis of silicon

[0180] A Raman spectrum of silicon included in the silicon-containing coating layer was measured using a 532 nm laser Raman spectroscopy for each anode active material prepared according to the above-described Examples and Comparative Examples. In the obtained Raman spectrum, a silicon peak intensity in a region of 515 cm^-1 wave number and a silicon peak intensity in a region of 480 cm^-1 wave number were measured. A peak intensity ratio (I(515)/I(480)) of the Raman spectrum of silicon was calculated by applying the measured peak intensities to Equation 4.

(4) Measurement of amorphous property and crystallite size of silicon

[0181] A crystallite size was calculated using an XRD analysis and Equation 3 as described above for each anode active material prepared according to the above-described Examples and Comparative Examples,

[0182] When the silicon particle size was too small to be substantially measured through the XRD analysis, the anode active material was determined as being amorphous.

[0183] Specific XRD analysis equipment/conditions are shown in Table 1 below.

[Table 1]

XRD(X-Ray Diffractometer) EMPYREAN
Maker	PANalytical
Anode material	Cu
K-Alpha1 wavelength	1.540598 □
Generator voltage	45 kV
Tube current	40 mA
Scan Range	10∼120°
Scan Step Size	0.0065°
Divergence slit	1/4°
Antiscatter slit	1/2°

[0184] Oxidation treatment temperatures, silicon oxidation number ratios, oxygen content ratios, crystallite sizes of silicon and Raman peak intensity ratios of the anode active materials according to the above-described Examples and Comparative Examples are shown in Tables 2 and 3 below.

[Table 2]

No.	oxidation temperature (□)	silicon oxidation number			oxygen content
		surface oxide layer	silicon-containing coating layer	OB/OS	surface oxide layer (at%)	silicon-containing coating layer (at%)	CB/CS
Example 1	100	3.3	1.4	0.42	25.2	8.5	0.34
Example 2	200	3.4	1.5	0.44	30.1	8.8	0.29
Example 3	300	3.5	1.9	0.54	32.5	13.8	0.42
Example 4	350	3.6	2.1	0.58	34.1	15.1	0.44
Example 5	80	2.9	1.5	0.52	22.0	8.2	0.37
Example 6	100	3.2	1.4	0.44	25.5	8.6	0.34
Example 7	100	3.3	1.5	0.45	25.7	8.5	0.33
Comparative Example 1	-	2.3	1.4	0.61	9.6	8.5	0.89
Comparative Example 2	400	3.9	3.0	0.77	34.3	20.2	0.59

[Table 3]

No.	silicon-containing coating layer
	crystallite size (nm)	peak intensity ratio (I(515)/I(480))
Example 1	amorphous	0.572
Example 6	7.1	1.181
Example 7	6.94	1.22

(4) Measurement of initial discharge capacity

[0185] Charging (CC-CV 0.1 C 0.01V 0.05C CUT-OFF) and discharging (CC 0.1C 1.5V CUT-OFF) were performed once as one cycle at room temperature (25 °C) for the lithium half-cells according to the above-described Examples and Comparative Examples to measure an initial discharge capacity.

(5) Measurement of initial capacity efficiency

[0186] 10 cycles of charging (CC-CV 0.1 C 0.01V 0.05C CUT-OFF) and discharging (CC 0.1C 1.0V CUT-OFF) were performed at room temperature (25 ° C) for the lithium half-cells according to the above-described Examples and Comparative Examples to measure a discharge capacity.

[0187] The discharge capacity at the 10th cycle was divided by the initial discharge capacity to calculate an initial capacity efficiency as a percentage.

(6) Evaluation on life-span property (capacity retention)

[0188] Each lithium half-cell manufactured according to the above-described Examples and Comparative Examples was charged with a constant current at room temperature (25 °C) at a current of 0.1C until a voltage reached 0.01V (vs. Li), charged with a constant voltage while maintaining 0.01V and cutting off at a current of 0.01C, and discharged with a constant current of 0.1C rate until the voltage reached 1.5V (vs. Li).

[0189] The charging and discharging were performed as one cycle, and charging and discharging of one cycle was further performed in the same manner. Thereafter, the applied current was changed to 0.5 C and 300 cycles were performed with a 10-minute interphase between the cycles.

[0190] The capacity retention was evaluated by calculating the discharge capacity after the 100 cycles as a percentage relative to the discharge capacity after the first cycle.

(7) Evaluation on gas generation

[0191] A slurry was prepared by uniformly mixing each anode active material of Examples and Comparative Examples and a CMC binder in a weight ratio of 97:3, and then 3 mL of the prepared slurry was injected into a syringe and sealed.

[0192] A total volume of the sealed syringe was 12 mL, and a volume of gas generated over time (1 to 7 days) was measured, and a gas generation amount was evaluated according to the following formula.



(V_a: a remaining volume inside the syringe (9 mL), V_b: a volume of gas generated from the slurry)

[0193] The evaluation results are shown in Table 4 below.

[Table 4]

No.	initial discharge capacity (mAh/g)	initial capacity efficiency (%)	capacity retention (%, 100cycles)	gas generation (%)
Example 1	1945	90.5	82.2	15
Example 2	1921	90.4	90.2	0
Example 3	1894	90.3	92.5	0
Example 4	1857	89.1	93.6	0
Example 5	1943	90.4	81.5	20
Example 6	1925	88.2	85.3	0
Example 7	1929	88.3	84.7	0
Comparative Example 1	1950	91.3	78.6	100
Comparative Example 2	140	50.2	15.9	0

[0194] Referring to Tables 2 to 4, the lithium half-cells of Examples had the silicon oxidation number ratio of 0.6 or less in the silicon-containing coating layer, so that the initial discharge capacity, capacity efficiency, and life-span property were generally improved compared to those from the lithium half-cells of Comparative Examples.

[0195] In Example 4, the oxidation number (O_B) of silicon included in the silicon-containing coating layer exceeded 2.0, and the initial discharge capacity and initial capacity efficiency of silicon were relatively lowered compared to those from Examples.

[0196] In Example 5, the oxidation number (Os) of silicon included in the surface oxide layer was less than 3.0, and the capacity retention was relatively lowered and the gas generation was relatively increased generated compared to those from other Examples.

[0197] In Example 6, the crystallite size of silicon included in the silicon-containing coating layer exceeded 7 nm, resulting in the relatively reduced initial capacity efficiency compared to those from other Examples.

[0198] In Example 7, the peak intensity ratio (I(515)/I(480)) of the Raman spectrum of silicon included in the silicon-containing coating layer exceeded 1.2, resulting in the relatively reduced initial capacity efficiency compared to other Examples.

Claims

1. An anode active material for a lithium secondary battery comprising a plurality of a composite particle, wherein the composite particle comprises:

a carbon-based particle containing pores therein;

a silicon-containing coating layer formed at an inside of the pores and/or on a surface of the carbon-based particle; and

a surface oxide layer formed on the silicon-containing coating layer, wherein the surface oxide layer contains silicon oxide,

and wherein a silicon oxidation number ratio defined by Equation 1 of the composite particle is 0.6 or less:

wherein, in Equation 1, O_B is an oxidation number of silicon included in the silicon-containing coating layer measured by an X-ray photoelectron spectroscopy (XPS), and O_S is an oxidation number of silicon included in the surface oxide layer measured by the XPS.

2. The anode active material for a lithium secondary battery of claim 1, wherein O_B is obtained by substituting a value obtained by subtracting 99.6 eV from a binding energy of silicon included in the silicon-containing coating layer measured by the XPS into a silicon oxidation number calibration curve, and
O_S is obtained by substituting a value obtained by subtracting 99.6 eV from a binding energy of silicon included in the surface oxide layer measured by the XPS into the silicon oxidation number calibration curve.

3. The anode active material for a lithium secondary battery of claim 2, wherein the silicon oxidation number calibration curve is obtained by connecting points corresponding to Si⁰, Si¹⁺, Si²⁺, Si³⁺ and Si⁴⁺ with a shortest distance between neighboring points in a graph in which an x-axis represents the oxidation number of silicon and a y-axis represents the value obtained by subtracting 99.6 eV from the binding energy of silicon measured by the XPS.

4. The anode active material for a lithium secondary battery of any one of claims 1 to 3, wherein a distance between a surface of the composite particle and the silicon-containing coating layer is 100 nm to 700 nm, and a distance between the surface of the composite particle and the surface oxide layer is 10 nm or less.

5. The anode active material for a lithium secondary battery of any one of claims 1 to 4, wherein O_B is in a range from 1.2 to 2.0 and O_S is in a range from 3.0 to 3.6.

6. The anode active material for a lithium secondary battery of any one of claims 1 to 5, wherein an oxygen content ratio defined by Equation 2 is 0.4 or less:

wherein, in Equation 2, C_B is a percentage (at%) of the number of oxygen atoms included in the silicon-containing coating layer relative to a sum of the number of atoms included in the silicon-containing coating layer and the surface oxide layer measured by the XPS, C_S is a percentage (at%) of the number of oxygen atoms included in the surface oxide layer relative to the sum of the number of atoms included in the silicon-containing coating layer and the surface oxide layer measured by the XPS.

7. The anode active material for a lithium secondary battery of claim 6, wherein C_B is in a range from 8 at% to 15 at%, and Cs is in a range from 15 at% to 34 at%.

8. The anode active material for a lithium secondary battery of any one of claims 1 to 7, wherein the carbon-based particle includes at least one selected from the group consisting of an activated carbon, a carbon nanotube, a carbon nanowire, graphene, a carbon fiber, carbon black, graphite, a porous carbon, pyrolyzed cryogel, pyrolyzed xerogel and pyrolyzed aerogel.

9. The anode active material for a lithium secondary battery of any one of claims 1 to 8, wherein the carbon-based particle has an amorphous structure.

10. The anode active material for a lithium secondary battery of any one of claims 1 to 9, wherein silicon included in the silicon-containing coating layer has an amorphous structure or a crystallite size measured by an X-ray diffraction (XRD) analysis of 7 nm or less,

wherein the crystallite size of silicon included in the silicon-containing coating layer is measured by Equation 3:

wherein, in Equation 3, L represents the crystallite size (nm), λ represents an X-ray wavelength (nm), β represents a full width at half maximum (rad) of a peak corresponding to a (111) plane of silicon contained in the silicon-containing coating layer, and θ represents a diffraction angle (rad).

11. The anode active material for a lithium secondary battery of any one of claims 1 to 10, wherein silicon included in the silicon-containing coating layer has a peak intensity ratio of 1.2 or less in a Raman spectrum defined by Equation 4:

wherein, in Equation 4, I(515) is a peak intensity of silicon included in the silicon-containing coating layer in a region of 515 cm^-1 wavenumber in the Raman spectrum, and I(480) is a peak intensity of silicon included in the silicon-containing coating layer in a region of 480 cm^-1 wavenumber in the Raman spectrum.

12. The anode active material for a lithium secondary battery of any one of claims 1 to 11, wherein the composite particle further comprises a carbon coating layer formed on an outermost portion of the composite particle.

13. A lithium secondary battery, comprising:

an anode comprising an anode active material layer that comprises the anode active material for a lithium secondary battery of any one of claims 1 to 12; and

a cathode facing the anode.

14. A method of preparing an anode active material for a lithium secondary battery, comprising:

firing a carbon-based particle including pores together with a silicon source to form a silicon-containing coating layer at an inside of the pores and/or on a surface of the carbon-based particle; and

heat-treating the carbon-based particle on which the silicon-containing coating layer is formed while injecting an oxygen gas to form a composite particle including a surface oxide layer formed on the silicon-containing coating layer, wherein the surface oxide layer contains silicon oxide,

and wherein a silicon oxidation number ratio defined by Equation 1 of the composite particles is 0.6 or less:

wherein, in Equation 1, O_B is an oxidation number of silicon included in the silicon-containing coating layer measured by an X-ray photoelectron spectroscopy (XPS), and O_S is an oxidation number of silicon included in the surface oxide layer measured by the XPS.

15. The method of claim 14, wherein the heat-treating is performed at a temperature from 100 °C to 250 °C to form the surface oxide layer.

Drawing